Technical Field
This invention generally relates to semiconductor circuit fabrication, and more specifically relates to programmable elements in semiconductor devices.
The proliferation of electronics in the modern world is due in large part to the development of integrated circuit semiconductor devices. Because these devices are designed and used for widely differing applications, it is often beneficial to have the ability to program the logic on a semiconductor device during its fabrication. Logic programmability of a semiconductor device involves changing or customizing the device circuitry to meet the requirements of a specific application. Semiconductors may also be constructed with duplicate circuit paths as another means of quality control. The redundancy afforded by the inclusion of duplicate paths may allow a device to function even when part of the chip is damaged, or when a part underperforms.
It is rarely cost effective to create a separate fabrication line, with customized masks and other associated fabrication features, for each type of semiconductor device, no matter how small the differences in their circuit requirements. It is much more efficient to produce a wide variety of devices intended for many different applications on a single fabrication line, and to build logic programmability into each device so that each can then be programmed to perform as needed for its particular application. In one customization technique, a programmable element such as a fuse or an antifuse may be programmed such that current pathways on the chip conform to the desired specifications. In one embodiment of this technique, an existing circuit path may be cut by blowing a fuse that has been placed in the path for that purpose. After the fuse has been blown, the circuit path of which it was a part no longer exists, and current is then directed along different pathways in the device. In another embodiment, a circuit path may be created where none existed before by programming an antifuse to carry current.
Programmable elements such as those discussed above represent one method for implementing redundancy and logic programmability in a semiconductor. Some conventional programming events, in which a programmable element is programed to carry current as needed in a chip, are fairly violent occurrences that result in substantial mechanical deformation of the element, In one conventional technique, for example, fuses constructed from metal lines are blown using a laser beam to evaporate the metal. This technique can cause damage to structures adjacent to the fuse, whether due to the spattering of molten metal, the inability to focus the laser beam only on the targeted fuse, unwanted reflections from the laser beam off the target fuse, or other reasons. The conventional solution to this problem of placing fuses far away from other structures on the chip is itself a problem because it leads to results such as increased device size, higher costs, and lowered efficiency. Electrically programmable fuses, or E-fuses, are programmed by applying a high voltage to them, and thus do not suffer from the problems affecting laser blown fuses. E-fuses may thus be closely spaced on a chip. Unfortunately, however, E-fuses suffer from reliability problems. For example, the blown metal in an E-fuse can oxidize in a moist environment, causing “grow-back.” The size of the metal pieces increases when the metal oxidizes, allowing them to connect and once again permit current flow.